relax strict getHome behaviour for LDAP users in a shadow state

* simplifies deletion process
* less strange behaviour when looking up home storage (as long as it is local)
* thus could enable transfer ownerships after user went invisible on ldap

backport of #17717

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

decouple userExists from userExistsOnLDAP check

allows to mark users as offline right away, avoids a gap of being not a
user and causing weird side effects

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

adjust tests

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

remove superfluous tests

- user_ldap is not exposed to public api, it is always behind ldap_proxy
- this is too much for a unit test
- integration tests cover userExists implicitly

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

ensure that only valid group members are returned

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>